The present invention relates to cutting elements with non-planar interfaces, and more specifically, to cutting elements with non-planar, non-linear interfaces.
Industrial applications such as subterranean drilling, cutting, machining, milling, grinding, and other highly abrasive operations require tools with a high resistance to abrasion, wear and percussion. In these instances, abrasive compacts or cutting elements designed specifically to withstand the highly abrasive operations are deployed.
Typically, these cutting elements are made up of facing tables with superhard abrasive layers mounted on substrates made from a less hard material. The substrate or mounting layer is typically made of a metal carbide material such as tungsten carbide, while the superhard abrasive layer is typically made of fine crystals of synthetic or natural diamond, cubic boron nitrite (CBN), wurzite boron nitrite, or combinations thereof. The cutting element is mounted on a carrier which is typically a cylindrical stud or post. One surface of the substrate is brazed to the carrier surface and the resulting stud is positioned in a socket in the body of the drill bit. Alternatively, the substrate itself may be of sufficient thickness as to substitute for the cylindrical stud to be directly received in a socket of the bit body.
The abrasive layer of the cutting element is typically created by a process known as sintered bonding or sintering where polycrystalline diamond (PCD) crystals are subjected to a combination of high pressure and high temperature processes, or alternatively, to chemical vapor or physical vapor deposition processes, such that an intra-crystalline bonding occurs. More details on the sintering process is disclosed in U.S. patent application Ser. No. 08/703,864, entitled "Curve Cutter with Non-Planar Interface", issued on Jan. 27, 1997 to John T. Devlin and assigned to the assignee of the present invention, herein incorporated by reference.
The bonding of the substrate to the polycrystalline material occurs under a temperature in excess of 1,300.degree. C. Subsequently, as the cutting element cools down, the substrate may shrink faster than the polycrystalline material layer due to differences in coefficients of thermal expansion. The differential shrinkage leads to residual shear stresses between the substrate and the PCD layer. As such, thermally-induced stress points may exist between the substrate and the table. In addition, local tensile stresses may be introduced into various regions in the outer cylindrical surface of the substrate and internally in the substrate. These stress points generally reduce the sintered bond strength of the polycrystalline material.
Further, during use, failures are often brought about by impact forces that release stress in the form of fractures in the compact. These fractures lead to a separation or a delamination of the polycrystalline material from the substrate material, as well as a fracture of the substrate during operation. Hence, the presence of stress points introduced during the formation and mounting of the cutting element, when augmented by stresses attributable to the loading of the cutting element during operation, may cause spallings, fractures, or delaminations of the diamond table from the substrate. These failure modes are likely to lead a complete failure of the PCD element.
Additionally, as the cutting element wears during use, a flat surface forms on an edge of the cutting element. As the flat formation increases in size due to wear, the carbide section of the cutting element receives greater stress and therefore wears at an accelerated pace. The wearing of the carbide support can decrease the cutting efficiency of the assembly and eventually results in a premature equipment failure. To overcome this problem, a thicker diamond layer has been specified to improve the operating life of the cutting element. However, as the thickness of the diamond table depends in part on the availability of bonding surface on the substrate, such attempts generally lower production yields and result in more expensive cutting elements.
A number of cutting element configurations have been developed to overcome the aforementioned problems. To improve the bond between the superhard material and the substrate, certain cutting elements have modified the shape of the superhard material's rear face from a flat configuration to a non-planar configuration to provide a better mechanical interlock between the superhard material and the substrate. Other attempts have focused on applying a non-planar interface (NPI) geometry such as ridges to increase the interfacial area between the superhard material and the substrate, as well as creating an interlocking mechanism between the materials. The ridges improve the bonding between the table and the substrate by accommodating distortions generated during the sintering process as well as the subsequent bonding of the cutting element onto a carrier. The distortions are generally caused by differences in coefficients of thermal expansion and elastic moduli between the superhard material of the facing table and the less hard material of the substrate.
Yet other NPI cutting elements employ one or more constant cross-sectional grooves or channels on the abrasive layer that communicate with corresponding channels or grooves on the substrate. However, the use of parallel grooves at the interface as a mechanical interlock is not ideal, for parallel ridges or similar perturbations may be less resistant to shear stresses in the direction of the grooves. This in turn requires that the cutter be properly oriented in the drill bit in order to realize the improved mechanical interlocking feature.
Other NPI cutting elements provide sinusoidal-like grooves that run perpendicular to the longitudinal axis of the cutting element. Additionally, some NPI cutting elements, as in U.S. Pat. No. 5,357,772, provide grooves that run radially to or in a circular fashion along the longitudinal axis of the cutting element. Yet other NPI cutting elements, as in U.S. Pat. No. 5,355,969, position concentric annular rings that expand outwardly from the center of the interface to increase available bonding surface area. In these elements, a wearing of the circular grooves and concentric annular rings causes a more rapid formation of wear flat on the cutting element and therefor reduces the cutting performance of the tool in formation drilling applications.
Certain other cutting elements use a curved or domed interface to increase the bonding area between the superhard material and the substrate. The domed interface increases the volume of the superhard material available for abrasive tasks. The domed surface for PCD formation is typically deployed with a transition layer between the substrate and the diamond layer, as discussed in U.S. Pat. No. 4,604,106. This is an attempt to reduce the differences in a co-efficient of thermal expansion (CTE) and an elastic modulus between the two materials. However, the spalling and delamination of the cutting element still exist. Moreover, the transition layer approach results in an inefficient sweep of a binder material from the substrate to the diamond and therefore impedes manufacturability.
The failure in the cutting element due to the aforementioned problems may necessitate a retooling of the drill bit in the field. When costs associated with equipment down-time, labor and replacement are considered, such failures are undesirable, especially in the case of deep-well and off-shore drilling applications.